Leviathan (/lɪˈvaɪ.əθən/; Hebrew: לִוְיָתָן, Livyatan) is a creature with the form of a sea serpent from Jewish belief, referenced in the Hebrew Bible in the Book of Job, Psalms, the Book of Isaiah, and the Book of Amos.
The Leviathan of the Book of Job is a reflection of the older Canaanite Lotan, a primeval monster defeated by the god Baal Hadad. Parallels to the role of Mesopotamian Tiamat defeated by Marduk have long been drawn in comparative mythology, as have been wider comparisons to dragon and world serpent narratives such as Indra slaying Vrtra or Thor slaying Jörmungandr, but Leviathan already figures in the Hebrew Bible as a metaphor for a powerful enemy, notably Babylon (Isaiah 27:1), and some 19th century scholars have pragmatically interpreted it as referring to large aquatic creatures, such as the crocodile. The word later came to be used as a term for "great whale", as well as for sea monsters in general.
The extinct sperm whale Livyatan melvillei is named after the Leviathan.